CHAPTER IX
PROPERTIES OF VULCANISED RUBBER
PHYSICAL PROPERTIES
VULCANISED rubber, as has been stated previously (see p. 102), is insoluble
or almost insoluble in all ordinary solvents for raw rubber. If it is heated
to comparatively high temperatures, 180 to 200° C.,for a long time disintegra-
tion takes place and a homogeneous solution is obtained; the rubber which is
obtained on reprecipitation with acetone is, however, very much reduced in
strength, and the passage into solution is obviously accompanied by a process
of degradation.
The swelling observed when vulcanised rubber is immersed in organic
liquids is much less pronounced than in the case of unvulcanised rubber.
The change in the swelling capacity of rubber with increasing degree of
vulcanisation has been investigated by Kirchhof,* who employed a mixture of
100 parts Plantation Para rubber and 12-5 parts of sulphur. This was heated
lor varying times to give a series of products having vulcanisation coefficients
of 1-2, 2-0, 3-5, 4*4 and 6-4 respectively. Discs 1 mm. thick were immersed
in different solvents, including light petroleum, benzene, carbon disulphide
and carbon tetrachloride, and the amount of liquid taken up in a given time
determined by weighing.
According to Kirchhof, the swelling reaches a maximum in twenty-four
hours, although the writer's experience is that a much longer time is necessary
for the completion of the process.
As in the case of raw rubber, different liquids have different swelling
powers for the same sample of rubber, the quantity of liquid taken up at
Maximum swelling being an exponential function of the specific gravity.
The maximum swelling with a particular liquid decreases as the coefficient
of vulcanisation of the rubber increases, and Kirchhoff deduces the following
quantitative relationship.
Qu. K€ = k (constant),
where Qu (Quellung) is the volume percentage of swelling medium taken up,
* Kolloidchem. Beihefte, 1914, 6, 1.
174.